Automate the benchmarks

[astrojuanlu]

Now that the workflow is more or less stable, we should try to automate running the benchmarks and publishing the result. There's a barebones cron.sh script but I don't think is up to date with current practices (i.e. having two separate checkouts of tests and results). There's a command to run only the new commits, as explained in the wiki, and we could probably do this once a week or once a day at most. This would require figuring out how to push to the results branch, or even better, how to create a pull request.

[astrojuanlu]

Perhaps take a look at https://github.com/asv-runner/asv-runner